It is already known to permanently anchor. a dental prosthesis in the jaw with the aid of titanium screws implanted in the jawbone. The screws are anchored in holes in the bone so that the upper part of the screw is situated level with or directly below the upper surface of the jawbone. The screw is then covered over with a flap of mucous membrane and is left unloaded for a rest period of 3 to 6 months in order for the bone to grow firmly to and form a unit with the implanted screw. After the rest period, the screw is exposed and a spacer element, preferably also of titanium, is arranged on the screw, after which a dental prosthesis is anchored on the spacer element. In this respect the dental prosthesis must be accurately adapted to the actual appearance of the jaw with the implanted titanium elements.
During the adaptation the dental technician works with a positive model of the patient's jaw, which model has been produced by taking a negative impression of the jaw with the aid of an impression spoon filled with impression compound. The hardened impression is then removed from the jaw and is filled with cast compound which is allowed to harden, a positive cast of the patient's jaw being obtained with projecting columns corresponding to the spacer elements of the implants projecting above the arch of the palate.
Swedish Patent 446,371 describes a method for facilitating the production of a model of the abovementioned type.
The position of the spacer elements is transferred to the impression with the aid of so-called guide elements (impression tops). These guide elements have a part which cooperates with the connection surface of the spacer element and a part for fixation in the impression material. The guide elements are secured on the spacer elements with guide pins (long screws). When the impression compound has hardened, the guide pins are unscrewed and the impression with the guide elements now set in is removed from the patient's mouth. Dummies of a geometry similar to that of the spacer elements are then screwed securely to the impression tops, and the impression is filled with modelling compound, usually plaster. When the plaster has hardened, the guide pins are unscrewed, and the impression with the guide elements is removed. A positive work model with molded-in dummies has now been obtained.
The method described in the above swedish patent can be used in cases where the spacer element comprises an internal thread in which the guide pin can be screwed down. However, other types of spacer elements which do not have an internal thread have recently been developed. Particularly in so-called single-tooth replacements, a type of spacer element is used whose base, in a similar manner to earlier spacer elements, is adapted to the upper part of the fixture and which has an upper, elongate narrower part of hexagonal design. There is no internal thread, and the impression top cannot therefore be secured with the aid of a guide pin.